G’ day,
My next project is to modernise the front of our house. The facade is porous in texture, clay brick with raked joints. What is the recommended specification of the cement and sand render mix? i.e. Ratio of mix, type of products, any additives. The finish is to be smooth, grainy, and will then be painted . 
Regards, Bob

Answer
Dear Bob,
The type of mix needed is directly related to the base to which it is to be applied, in this case porous textured clay masonry with raked joints.  Additionally some consideration should be given to the atmospheric conditions to which the render will be exposed.  By that I mean proximity to the ocean or heavy pollution from chemicals.

The materials used for render are the same as for the mortar used to lay bricks.  The recommended mix for porous or weaker backgrounds externally is 1:1:6, (i.e. 1 part cement, 1 part lime and 6 parts sand).  This is a medium strength mix for porous or weaker backgrounds.

If you find that the brickwork is not as porous as you first thought, you could try a 1:½:4½ mix.  This is a stronger mix recommended for stronger backgrounds.

Always do a small trial mix before getting stuck into the real thing, particularly if you are using colouring agents in the mix.  This enables you to see the finished product, colour, cracking, porosity etc., and adjust it to suit your situation.

The materials to be used should be stored somewhere where they will not collect contaminants and should generally be covered before use.  Long storage of lime and cement will generally see the material deteriorate and should be avoided.  The following is a quick guide to the requirements for your materials:
•Cement
•gives the render its strength and the type doesn’t really matter.  It can be:
•GP, general purpose ;or GB, general purpose blended, or
•Use off white cement for a light coloured render
•Lime
•gives the mix its workability and is either hydrated or building type.
•Some operators recommend soaking the lime in equal parts of water for 24 hours prior to use, to improve its performance.
•Water
•should be good enough to drink
•use enough to make the mix stiff.
•The more water you use the weaker the mix will be
•Sand
•Should be free of clay deposits and vegetable matter.  A good test is to rub it in the palms of your hand and see whether they get dirty or whether it just leaves a coloured residue. 
•If you need to apply two coats, the base coat should be coarse graded sand and the finish coat should be a fine graded sand.
•If the sand is too fine however, it will not be easy to apply and will fall off as if it were too wet
•Additives
•Generally there are none required
•If you are going to colour the render, in preference to painting it you can use a colouring agent such as oxides.  These are generally applied at a rate of about no more than 10% of the cement content.

The number of coats required will depend on whether the base brickwork is sound and in good alignment.  If either are poor or you live in close proximity to the ocean I would recommend doing the job in two coats.

The first coat can be either applied to a thickness of about 10 mm to 15 mm maximum.  If you use this system, once the render becomes firm, you need to scratch it to roughen it up ready to receive the final coat. 
Alternatively you could apply a dash coat, which is splattered or flicked onto the brickwork to give a rougher surface to accept the finish coat.  This dash coat should be a mix of 1 part cement and 2 parts sand.  In this case the finished coat should be around 10 mm thick.

When you have applied the render you will need to allow three days for it to “cure” or gain its strength before any further work can be done.  Additionally you must not let the render dry out too quickly.  For this reason it is advisable to cover it with plastic to allow it to dry or cure slowly and at an even rate.  Failure to adhere to this will lead to wholesale cracking and failure.

If you have large areas to be covered, have weepholes for sub floor ventilation or your brickwork has expansion joints: you will need to take care not to cover these construction aspects or consider using control joints to avoid large amounts of uncontrolled cracking. 

Finally, don’t forget the safety issues, to yourself and the job.  Use protective clothing such as gloves, because cement and lime are corrosive to the skin.  Don’t forget to also protect your windows, doors and external timbers from the render and spattering as it will save having to clean up later.  And please use a proper scaffold when working off the ground.

Hope this helps

Kind Regards,
Mike